This invention relates to an apparatus for detecting a knocking of an internal combustion engine from the light emitted during combustion in a cylinder of the internal combustion engine.
The deep correlation between ignition timing and the internal pressure in an engine cylinder is generally known. The internal pressure of the cylinder caused by the explosion of air-fuel mixture in the absence of a knocking does not carry a high frequency (normally a frequency component from 5 to 10 KHz and 11 and 15 KHz, which is a component in a frequency range determined by the bore diameter of the engine cylinder involved and the sound velocity at the time of combustion, and is caused by intermittent rapid combustion) but begins to carry such a high frequency near the maximum value of internal pressure in the presence of a knocking, resulting in a vibration or a noise being produced outside of the cylinder. A close look at the internal pressure signal generated in the cylinder or the manner in which the vibration or noise is generated out of the cylinder shows that at the initial stage of knocking (we call this trace knock), the high frequency component begins to occur at the crank angle associated with the maximum internal pressure, and when it develops into a larger knocking (we call this light or heavy knock), the high frequency component occurs at the crank angle advanced from the maximum value of the internal pressure. If the ignition timing is controlled by accurately detecting the vibration and noise generated out of the cylinder by a knock and by feeding back the result of a detection, therefore, the engine efficiency may be remarkably improved. There has not yet been developed any detector or sensor, however, which is capable of detecting accurately the knocking making up a feedback factor and is stably operable in adverse environments.
Conventional knocking detectors detect a knocking by detecting a vibration or a noise generated outside of the cylinder as described above. In the method of detecting a knocking outside of the cylinder, however, it is difficult to discriminate the vibration or noise of a high frequency caused by the knocking from the vibration or noise caused by the structure of the internal combustion engine or the automotive vehicle involved. Such a method thus poses a great problem of the need of a complicated discriminator circuit. A method of detecting a knocking by use of the pressure in the cylinder, on the other hand, is advantageous against external disturbances but has the disadvantage that the apparatus is exposed to a high temperature and a pressure sensor capable of stable operation in a rigid environment of vibrations is lacking.